If she needs a manual one; wrap the cuff around the upper arm; (make sure it is deflated) take your stethoscope and place it on your inner arm. Pump the cuff to 200 and slowly deflate it listening to a beat. The first beat is the top number keep deflating till you can't hear no more beats. That is your bottom number.
Hope I explained this easily
But those wrist ones and others are very accurate too

If she needs a manual one; wrap the cuff around the upper arm; (make sure it is deflated) take your stethoscope and place it on your inner arm. Pump the cuff to 200 and slowly deflate it listening to a beat. The first beat is the top number keep deflating till you can't hear no more beats. That is your bottom number.
Hope I explained this easily
But those wrist ones and others are very accurate too

FYI
if you can't hear the beats... carefully look at the cuff and once you see the needle do a movement (jump like ) is the top number and when it stops it's the bottom number.
This is good is the dr's office if you are interested in comparing your findings to theirs.